Rather than list distinct features (since I do not know what they could be), I will use Oleg Chudnovsky’s words because he nailed it succinctly:

““The most powerfully growing products,” he says, “do three things at once: they make you look smart to the people you invite. They give real value to you when the people you invite join. And they give real value to the people you’ve invited once they sign up.”

G-Mail, Twitter, Snapchat, Facebook,  Uber, etc